1. Field of the Invention
The present invention relates to an optical unit including an optical fiber preferably applied to various uses such as an oil well sensor for instance, which require thermal resistance.
2. Description of the Related Art
Optical fibers including optical fibers are used in various uses such as temperature sensors. To use optical fibers in high-temperature environments such as oil wells, high thermal resistance is needless to say required.
Applied to optical units in practical use so far are base fibers each comprised of an optical fiber made of glass or resin with a sleeve made of any heat-resistant resin such as polymethylmethacrylate (PMMA) covering the periphery thereof. Frequently a plurality of such base fibers are bundled together, enclosed with a metal tube, metal wires or a resin tape wound therearound into one or more layers, and further covered with a proper sheath. More specifically, heat-resistant optical units in general have multilayer structures.
Thermal expansion related to temperature change, or related to bend an optical unit, sometimes causes so-called “micro bend” because side pressure is given to base fibers in the optical unit. A micro bend given to base fibers is one of primary causes for increasing transmission loss. Optical base fibers of a graded index (GI) type are more susceptible to transmission loss increase induced by such micro bend caused by side pressure than optical base fibers of a single mode (SM) type. More specifically, optical units containing optical base fibers of a GI type require special structural measures against side pressure. Such structural measures, as well as the aforementioned multilayer structures, lead to enlargement of optical units in diameter.
In many practical uses, it is required to wire a large number of, or a great amount of, optical units and thus respective diameters of the optical units strongly dominate the total scale of the system of wiring. Therefore, enlargement of optical units in diameter is a technical problem to be solved.